1. Field of the Invention
The present invention relates generally to a method of optimizing an output signal of an optical receiver using FEC and an optical receiving system using the method, and more particularly to a method of optimizing the output signal of an optical receiver using FEC and an optical receiving system using the method that can maintain an optimized output signal of the optical receiver at an optimum level using the FEC in an optical communication system.
2. Description of the Prior Art
In general, an optical communication system that converts data into optical signals and transmits the optical signals through an optical cable at high speed employs a Forward Error Correction (FEC) method so as to correct errors generated during transmission of the optical signals.
The FEC method is an error correction method that is used in applications necessarily requiring real-time transmission. The FEC method transmits redundancy data together with data in order to recover original data, and recovers the original data using the redundancy data when received data are damaged.
FIG. 1 is a block diagram showing a schematic construction of an optical communication system using FEC. The optical communication system includes an FEC encoder 2, a data modulator 3, an optical transmitter 4, an optical cable 5, an optical receiver 6, a clock and data recovery unit 7, and an FEC decoder 8. The FEC encoder 2 encodes optical transmission data 1 in one of various formats such as SONET (Synchronous Optical NETwork), Synchronous Digital Hierarchy (SDH), Internet Protocol (IP), gigabit Ethernet, and Asymmetric Transfer Mode (ATM) formats. The data modulator 3 modulates data output from the FEC encoder 2 to be transmitted. The optical transmitter 4 converts transmission data output from the data modulator 3 into optical signals and transmits the optical signals. The optical cable 5 provides a path through which the optical signals transmitted from the optical transmitter 4 are passed. The optical receiver 6 converts the optical signals transmitted through the optical cable 5 into electric signals. The clock and data recovery unit 7 recovers a clock and data from the electric signals output from the optical receiver 6. The FEC decoder 8 corrects transmission errors of the data recovered by the clock and data recovery unit 7.
Referring to FIG. 1, an error measurement equipment 9 is a means for outputting a Bit Error Rate (BER) calculated in the FEC decoder 8.
In the optical communication system described above, data S4 transmitted in the form of an optical signal through the optical cable 5 are distorted due to optical loss, the nonlinear effects of an optical line like as optical dispersion of optical fiber, and optical noise factor generated from erbium-doped fiber amplifiers. The optical signal distorted during transmission is compensated for its distortion in various fashions. In general, an optical amplifier is employed to compensate for distortion caused by the optical loss, and an optical dispersion compensator is employed to compensate for distortion caused by the optical dispersion. However, there is no way to compensate for distortion caused by the nonlinear effect, so distortion of the optical signal caused by the nonlinear effect increases the BER.
FIG. 5a is a block diagram showing a general construction of an optical receiver 6 having a distortion compensation function. The optical receiver 6 includes an electro-optical converter 61, a post-amplifier 62 and a limiting amplifier 63. The electro-optical converter 61 converts transmitted optical signals into electric signals. The post-amplifier 62 amplifies the electric signals output from the electro-optical converter 61. The limiting amplifier 63 amplifies the electric signals output from the post-amplifier 62 to electric signals “1” or “0” and outputs the electric signals “1” or “0”.
FIG. 5b is a view showing signal output characteristics 151 to 153 and probabilities of occurrence of errors 160 to 162 with respect to variations of a reference voltage S12. When the reference voltage S12 is at an optimum level, the output signal of the limiting amplifier 63 exhibits a symmetric characteristic as indicated by reference numeral 152, and has a minimum distribution of probabilities of occurrence of errors as indicated by reference numeral 161 with probabilities of occurrence of errors for bits “1” and “0” being equal to each other.
In contrast, when the reference voltage S12 is at an excessively low or high level, the output signal of the limiting amplifier 63 exhibits an asymmetric characteristic as indicated by reference numerals 151 and 153, and has a broad distribution of probabilities of occurrence of errors as indicated by reference numerals 160 and 162 with one of probabilities of occurrence of errors for bits “1” and “0” being greater than the other.
As a result, in order to reduce a probability of occurrence of a bit error, the reference voltage is required to have an optimum level.
In the optical communication system, optical signals transmitted through optical amplifiers and optical cables undergo phenomena in which the optical signals are compressed or spread due to the dispersion and nonlinear effects of an optical cable and noise is added to “1” level signals of the optical signals due to the naturally emitted noise of an optical amplifier. Therefore, in order to obtain optimal data characteristics by judging levels of signals to be levels “1” or “0” in the clock and data recovery unit 7, that is, a minimum BER, it is necessary to control the distributions of probabilities of errors for levels “1” and “0” of electric signals output from the optical receiver 6.
However, since in the prior art, a reference to judge levels of signals to be levels “1” or “0” is fixed, variations in the intensity of received optical signals or in judging level according to the eye-diagram of transmitted optical signals cannot be taken into account.
U.S. Pat. No. 5,146,079 entitled “Broadband optical receiver with active bias feedback circuit” discloses an optical receiver that is capable of minimizing distortion and a Signal-to-Noise ratio (SN). The patented optical receiver is provided at its reception stage with an attenuator and controls an attenuation ratio on the basis of the feedback of the levels of received signals, so the optical receiver can monitor and warn of the loss of signals using a signal loss monitor while maintaining the output levels of analog received signals to be constant. The patented optical receiver achieves output of a certain level at an analog signal stage and monitors only the loss of signals. Accordingly, the patented optical receiver does not disclose a solution to the distortion of signals.